1. Field of the Invention
This invention relates to a positive resist of the type used in the fabrication of microcircuits, and particularly to a positive resist of poly(methacrylic anhydride).
2. Description of the Prior Art
Advances in modern electronics have produced the well known silicon "chip." This silicon chip contains all the elements of an electronic circuit such as transistors, capacitors, etc., which are interconnected by metallic strips on the surface of the chip. Reduction in circuit size has reduced the cost per function of these chips and increased the switching speed of the various logic functions of the circuits.
These chips are produced using photolithographic techniques, including electron-beam lithography. Such photolithographic techniques are discussed in a paper entitled, "Electron Radiation of Polymers and its Application to Resists for Electron-Beam Lithography" by M. J. Bowden, published in CRC Circuit Review Solid State Materials Science, 1978 (published 1979) 8223-64.
Briefly, the photolithographic process calls for the semiconductor surface of the wafer substrate to be coated with a polymeric resist. The coated substrate is then exposed in selected areas to a radiation source such as ion beam, X-ray, or electron-beam, any of which upon striking the resist changes the chemical properties of the resist. One material which has been utilized advantageously as a positive resist in the electron-beam lithographic process is poly(methacrylic anhydride) (PMAH).
The surface of the semiconductor wafer must be coated with the PMAH film prior to exposure to the radiation source such as electron beam. However, the solvents for PMAH, for example, dimethylacetamide, dimethylformamide and N-methylprrolidione do not adequately wet the semiconductor wafer surface, particularly silicon, to provide a uniform coating. Thus, the PMAH film is currently prepared by first applying a solution coating of poly(t-butyl methacrylate) (PTBMA) to the surface of the wafer. PTBMA is then heated to above 200.degree. C. for two to three hours to convert the coating to a film of PMAH. The problem with this method is that there is significant variation in the composition and consistency of the resist produced, and quality control is difficult. There can be incomplete conversion of the PTBMA to PMAH, depending on the uniformity of process conditions, particularly temperature within the process oven. As a result, there are areas of the film coating which can contain both PTBMA and PMAH. During subsequent steps in the photolithograpahic process, the irradiated resist area is dissolved, and thus removed from the wafer surface, upon exposure to development solvent. Since the PTBMA in the nonirradiated areas is more soluble in these development solvents than the PMAH in the same areas, the resist film structure remaining on the wafer surface may be uneven and inconsistent. In addition, the reaction to convert PTMBA to PMAH results in the liberation of isobutylene and water by-products. Although these by-products are in gaseous form under the reaction conditions, bubbles of the gases can remain trapped in the polymeric film coating, creating areas of varying continuity and integrity. Control of the reaction parameters in order to produce a consistent resist product under the conditions described above is very burdensome and frequently inadequate. This invention is concerned with a method which permits at least the majority of the PMAH positive resist to be applied directly to the prepared semiconductor wafer surface, so that the above-described problems can be avoided.